The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the reduction of errors in NMR images caused by motion resulting from pulsatile blood flow.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the gyromagnetic constant γ of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomena is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Object motion during the acquisition of NMR data produces both blurring and “ghosts” in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, each view is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views such as that produced by a patient moving, by a respiration or a cardiac cycle, or by peristalsis, is referred to hereinafter as “view-to-view motion”. Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to hereinafter as “in-view motion”.
Both blurring and ghosting can be reduced if the data acquisition is synchronized with the functional cycle of the object to reduce view-to-view motion. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object “looks” the same in each view. In the case of motion caused by the pumping action of the heart, it is common practice to acquire an ECG signal that indicates cardiac phase. This signal can be used to gate the acquisition of NMR data only during certain phases of the cardiac cycle.
Functional magnetic resonance imaging (fMRI) of the brain is performed by acquiring a time-series of images of one or more anatomic sections of interest. As described, for example, in U.S. Pat. No. 5,603,322, algorithms used to extract functional activity from an fMRI time-series are based on the assumption that signal intensity fluctuations in each voxel are dependent only on physiologic changes induced by a functional activation task of known timing. However, image-to-image global head motion invalidates this assumption. Motion occurring over the acquisition of the time-series of images can be conceptually divided into two categories: intra-image motion and inter-image motion. Intra-image motion refers to view-to-view motion which occurs during the period of time that the phase-encoded lines of data necessary to reconstruct a single image are being acquired. This motion can originate from i) respiratory expansion and contraction of the thorax, ii) cardiac-driven pulsatility of the brain, overlying vessels, and cerebral spinal fluid, and iii) bulk rotational and translational head motion. Intra-image motion results in image blurring and ghosting. However, for fMRI studies performed with a single-shot EPI acquisition in which all data for one image are collected in under 100 msec, intra-image (view-to-view) motion is not a major problem. Inter-image motion is defined as that occurring between acquisition of successive images in an fMRI time-series. Such inter-image motion usually arises from global head movement or tissue motion due to the pulsatile flow of blood as it is pumped into the brain by the heart.
It is common practice to acquire navigator signals during an fMRI scan to correct for inter-image motion caused by global head motion. Navigator pulse sequences such as those disclosed in U.S. Pat. Nos. 4,937,526, 5,539,312 and 6,958,605 are interleaved with the fMRI pulse sequences and the data they acquire is used to correct for global head motion.